In the foundry art, cores or molds for making metal castings are normally prepared from a mixture of an aggregate material, such as sand, and a binding amount of a binder or binder system. Typically, after the aggregate material and binder have been mixed, the resulting desired shape or pattern is then cured with the use of catalysts and/or heat to a solid, cured state.
In the usual foundry operation, hot molten metal is poured around the sand core and solidifies with the cavity in the casting taking on the form of the sand core. Because of the high temperature of the metal, the resin system slowly burns out, removing the resin binder from the system. As will be appreciated by those skilled in the art, once the resin binder has been burned out or decomposes, the core, formed essentially of sand, becomes free flowing and can be readily poured out of the casting. If, on the other hand, the binder does not degrade to a sufficient extent, the core, or a portion thereof, remains inside the casting and must be removed by mechanical means.
Iron and steel castings are generally poured at high temperatures where burnout of the resin binder is usually complete. However, metals such as aluminum and brass having lower melting points are poured at lower temperatures where the resin binders frequently do not burn out completely, with the result that cores, or portions thereof, are frequently left in the casting. It is difficult, if not impossible, to shake the sand out of such castings.
In addition, it has not been possible to use resin-bonded sand cores when making cast or molded plastic parts. The temperature needed to burn out the resin binders from such cores is so high that the plastic parts would be destroyed in any attempt to burn out the binders.
The use of sand cores to make complex casting shapes with interior passageways or holes in plastic parts would greatly reduce the cost of manufacturing such parts. In the past, these have been made in two halves and welded or clamped together. This adds to their assembly cost and is impractical for complex shapes. It would therefore be desirable to have a method to break down resin bonded sand cores at relatively low temperatures. Such a method should weaken the cores and permit the sand to shake out at temperatures below the decomposition or melting point of the plastic and preferably below the glass transition temperature of the plastic. Such a method would also be useful with castings of lower-melting metals such as aluminum and brass.
One method to improve the shakeout of sand from metal castings is disclosed in U.S. Pat. No. 2,597,896 to Oster. The casting containing the sand core is exposed to steam under high pressure and then the pressure is suddenly released. Water within the core tends to flash instantaneously to steam with a sharp explosion that tends to disintegrate the core material and to discharge it forcibly from the casting.
U.S. Pat. No. 4,620,586 to Musschoot discloses placing a mold flask containing the sand and hot metal into a vacuum chamber where the pressure is reduced to a very low value. This reduction of pressure causes the moisture in the sand to flash into vapor. It is said that this process causes the sand to fall away from the casting. The sand molds contemplated include a binder material; however, the specific binder is not disclosed.
U.S. Pat. No. 3,563,711 to Hammond, et al. discloses a process for removing siliceous cores from metal castings. The castings containing the cores are heated with a concentrated alkaline solution under pressure and then the vapors are released from the vessel to reduce the pressure and induce boiling of the solution. The pressurization and release steps are repeated until the core is completely removed.
U.S. Pat. No. 4,540,467 to Grube, et al. discloses a method and apparatus for fragmenting municipal waste material which is said to be applicable also for the removal of core material from metal castings. The material is heated under pressure and then the pressure is released suddenly to cause water in the material to flash to steam. The resulting sudden expansion within the material causes it to fragment. It is said that bases such as saturated sodium hydroxide solution could be used to soften compacted sands before they are subjected to this treatment.
U.S. Pat. No. 1,304,200 discloses an apparatus which projects a water jet into a rolled metal bar to remove the sand core from the bar.
U.S. Pat. No. 3,046,147 to Hathaway, et al. discloses a water soluble core binder prepared from borax and phosphate which is said to be useful for making aluminum and plastic castings. They recognize the problems associated with using conventional binders for sand cores when making plastic castings
Japanese patent publications 52-11127 and 52-50923 disclose cores for precision metal castings. The first uses a high temperature sintered siliceous core which is removed by dissolving in alkali. The second uses a core binder containing calcium chloride, bentonite and starch that gives a core which is said to be readily collapsible in water.
British Patent No. 876,033, published Aug. 30, 1961, discloses that silica containing products bound with resins lose some of their flexural or compressive strength when exposed to moisture. Silane compounds were added to the resins to improve the moisture resistance of materials bound with the resins.
We have now discovered a method for removing sand cores from castings which have been made under conditions that do not cause burnout of the resin binder. This method is particularly applicable to removing sand cores from cast or molded plastic parts. Such parts are steadily replacing metal parts in automobile engines and similar applications. It is also useful for removing sand cores from other castings such as aluminum and brass which are prepared at temperatures that do not thermally degrade the resin core binders.